Inertial confinement fusion requires driver energies of a few MJ with a power in excess of 100 TW and must be focusable onto an area less than .about.1 cm.sup.2. All driver concepts presently under study have in common that they deliver the energy in form of a beam. Five of these driver concepts are (1) laser beams, (2) relativistic electron beams, (3) light ion beams, (4) heavy ion beams and (5) macroscopic projectiles. Of these drivers only relativistic electron and light ion beams can be cheaply produced, but this advantage is offset by the difficulty to meet with them the stand-off requirements for the target from the walls inside a thermonuclear reactor. Heavy ion beams, but even more so laser beams, both of which easily meet this requirement, have the drawback to be very expensive. Macroscopic projectiles also easily meet this requirement but should be also rather expensive if the minimum needed velocity is about .about.200 km/sec. Even though all previous studies on impact fusion.sup.(1) have suggested a minimum velocity of this magnitude, the fast liner approach to nuclear fusion.sup.(1,2,3,4,5) shows that substantially smaller velocities can be used for magnetized, less than solid density, targets and for which only .about.20 km/sec may suffice. Rather than producing these velocities by liner implosion one may generate them separately from the plasma by a mass accelerator and to let a projectile make an impact upon a magnetized target. Studies on this concept were made by Ribe and Vlases.sup.(1) and also by Tidman and Goldstein.sup.(5). In this concept however, the much lower target density results in a comparatively low thermonuclear gain. Therefore, unless the target is made rather large, no net energy gain can be obtained.
This drawback can be overcome in the novel approach suggested here. In this approach again a .about.20 km/sec projectile also serves here to ignite a thermonuclear reaction in a magnetized, less than solid density, target, but the energy released in this low density low yield target is now used to ignite a second high density high yield target. This two stage target promises very large final thermonuclear yields even with a comparatively low initial impact velocity. Furthermore, the required impact velocities are here so snall that they can be cheaply produced by magnetic acceleration or isentropic light gas guns.sup.(1).